8/18/2022 10:07 PM | |
Joined: 1/21/2013 Last visit: 10/8/2024 Posts: 3164 Rating: (490) |
Hi Dkrills, it would be great if you could explain the reason for your question. It is then easier to explain the functions from the appropriate point of view. Do you want to know why they are used or how? A short explanation for the reason why SFC 14 "DPRD_DAT" and SFC 15 "DPWR_DAT" are used: For the PLC (especial for S7-300/400) it's only possible to read or write data in size of BYTE (8 bit), WORD (16 bit) or DWORD (32 bit). If you have to read/write data in different size, you have to combine some read/write commands (e.g. for writing 3 bytes you can write 1 word followed by 1 byte). As long as the accessed data is inside the process image, you can do this without the SFCs. If the data is outside the process image, you would need peripheral access (PIB, PIW, PID or PQB, PQW, PQD). But, since the PLC cycle is independent of the bus cycle, for the example above it's possible you write the word to cycle n and the byte to cycle n+1. That means the 3-byte-value would have the wrong value for one cycle. The reaction of a device to this wrong value might be dangerous. SFC 14 "DPRD_DAT" and SFC 15 "DPWR_DAT" are used to read/write data consistently, so the situation described above would never happen. Maybe the FAQ "How do you read consistent data from a DP standard slave/PROFINET IO device and write it consistently to a DP standard slave/PROFINET IO device?" and the linked documents would answer a few more questions. When working with an S7-1200/1500 "DPRD_DAT" and "DPWR_DAT" are less important because these PLCs have a process image which is covering the whole address range. With default address configuration, all data are transferred via process image and therefore consistent. Kind regards |
This contribution was helpful to2 thankful Users |
8/20/2022 3:18 PM | |
Joined: 9/27/2006 Last visit: 10/8/2024 Posts: 12284 Rating: (2685)
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Hello Dkrills: Additionnally, it would be best if you told us more about the drives you want to control using SFC 14/15. Siemens Micromaster and Masterdrives (VC ad MC) could use SFC 14/145 for certain operations, certain Danfoss drive models also. See the description of the PKW mechanism in thus entry: https://support.industry.siemens.com/cs/ca/en/view/8894584 Many drive manufacturers do not include the functionalities of SFC 14/15 in their drive controls. Siemens' newer Sinamics drives use specially designed telegrams, which precludes the necessity of transfering data consistetly (which is what SFC 14/15 do). https://support.industry.siemens.com/cs/ca/en/view/28991143 (please read the first entry Definition of terms, to understand what "consistent data" refers to). There are many discussions and documents on the forum considering the transfer of consistent data to a slave drive, they should give you a clearer idea of why and when you need to use SFC 14 and SC 15. For example: https://support.industry.siemens.com/forum/ca/en/posts/sfc14-and-sfc-15/218115/?page=0&pageSize=10 Hope this helps, Daniel Chartier |
Last edited by: dchartier at: 08/21/2022 01:45:24 |
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3/20/2024 12:47 PM | |
Joined: 2/13/2024 Last visit: 3/20/2024 Posts: 1 Rating: (0) |
When you say "...it's possible you write the word to cycle n and the byte to cycle n+1...", did you mean bus cycle or plc cycle? |
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